This invention relates to a method for connecting a conductive member formed on a board (e.g., a flexible wiring board and a glass board) and an electrode of an electronic component mounted on a board (e.g., a printed. circuit board).
Recently, not only household electric appliances but also apparatus for industrial use have been made compact in size, which is realized by mounting a great number of electronic components on a single board.
There are various means known in the art for connecting electrodes of different boards, such as soldering, conductive rubber connecting, and welding with pressure (hereinafter called the pressure welding) making use of an anisotropic conductive sheet.
A conventional connecting procedure of connecting a conductive member provided on a flexible wiring board and an electrode of an electronic component mounted on a printed circuit board will be described below. In such a conventional connecting procedure, a soldering process and a pressure welding process employing an anisotropic conductive sheet are used.
FIG. 13 cross-sectionally illustrates a conventional way of connecting a conductive member provided on a flexible wiring board with an electrode of an electronic component mounted on a printed circuit board by means of a pressure welding process using a thermosetting resin capable of shrinking in volume. FIG. 14a relates to a soldering process. FIG. 14b relates to a pressure welding process using an anisotropic conductive sheet.
As shown in FIG. 13, a flexible wiring board 1 is constructed by a total of five layers. More specifically, a conductive member (not shown in the figure) is formed on a flexible base film 2 via a first adhesive layer 4a. Formed on an electrode via a second adhesive layer 4b is a cover film 3. An area of the flexible wiring board 1 not covered with the cover film 3 acts as a lead section (not shown in the figure). This lead section is aligned with an electrode of a printed circuit board 7. Then, the lead section and the electrode are pressure-welded together. Thereafter, a resin that is volume-shrinkable is injected between the flexible wiring board 1 and the printed circuit board 7, to form a thermosetting resin layer 9. The thermosetting resin layer 9 exerts a shrinkage force thereby enhancing the mechanical connection of the flexible wiring board 1 and the printed circuit board 7 as well as the electrical connection between electrodes.
In the soldering process (see FIG. 14a), a lead section 6 of the flexible wiring board 1 is aligned with an electrode 8 of the printed circuit board 7. Then, a joint layer 26 of solder is formed to mechanically connect the flexible wiring board 1 and the printed circuit board 7 and to electrically connect the lead section 6 and the electrode 8. Generally, the lead section 6 is formed by solderplated copper.
In the pressure welding process (see FIG. 14b), the lead 6 of the flexible wiring board 1 is aligned with the electrode 8 of the printed circuit board 7. Then, an anisotropic conductive sheet is provided between the boards. Pressure and heat is applied to the flexible wiring board 1. An adhesive layer 27 is formed to mechanically connect the flexible wiring board 1 and the printed circuit board 7. At the same time, the lead 6 and the electrode 8 are electrically connected together through a conductive particle 28 contained within the anisotropic conductive sheet.
Each of the above-described techniques, however, produces some problems.
In the connecting technique using volume shrinkage force exerted by a thermosetting resin, a heating process is required. This means that the flexible wiring board 1 must have resistance to application of heat. To make the flexible wiring board 1 heat-resistant, the cover film 2 and the base film 3 must be formed by polyimide which is an expensive material.
In the connecting technique by means of soldering, a step of removing a flux by washing is required. Additionally, lead is toxic to the human body and use of lead produces environmental problems. Further, soldering provides a poor connecting strength, so that the flexible wiring board 1 and the printed circuit board 7 cannot be connected together sufficiently. Furthermore, solder must be heated and the cost of material increases because of the above-described reason.
In the connecting technique using an anisotropic conductive sheet, a fillet with strong adhesion cannot be produced, since an anisotropic conductive sheet is supplied in the form of a sheet. This results in providing poor tear-off resistance between the adhesive layer 27 of the anisotropic conductive sheet and the flexible wiring board 1 (the printed circuit board 7). Therefore, the flexible wiring board 1 and the printed circuit board 7 are not connected together sufficiently. Additionally, the anisotropic conductive sheet must be heated and the cost of material increases because of the same reason as described above.
Japanese Patent Application, published under Pub. No. 4-82240, shows a technique. This technique is as follows. In a step of connecting electrodes of two boards, one of the boards is placed on a platform formed by an ultraviolet-transmissive material. Electrodes of the boards are aligned with each other. The other board is placed on the one board. A photosetting resin is injected between the boards. The injected photosetting resin is irradiated with UV from under the platform, to connect the two boards at room temperature. This prior art technique, however, has the problem that a lower of the two board must be formed by an ultraviolet-transmissive material. The range of applications is thus be limited.